Arthropoda — the biggest phylum

This is the phylum holding the largest number of species in Kingdom Animalia. About 75% of all animal species belong to this group.

The class holding the most species within Arthropoda is the insects — about 950,000 species belong to it.

Butterfly Honeybee Grasshopper Mosquito Spider Scorpion Centipede Prawn Crab

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Figure 13.22 — several species of Arthropoda: butterfly, spider, scorpion and centipede.

They live in terrestrial, sea water and fresh water environments.
Triploblastic and coelomate, with jointed legs — which is why they are called arthropods.
The body is segmented, and the segments combine into tagmata (head, thorax, abdomen).
The body surface bears a chitinous cuticle, which acts as an exoskeleton.
Some species bear wings.
The body shows bilateral symmetry.
There are female and male animals — that is, they show sexual dimorphism.

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Figure 13.23 — the external form of an insect: head, thorax, abdomen and jointed legs.

Assignment 13.2

Collect the dead insects found around the house and prepare an insect box.

What the exoskeleton costs

A chitinous cuticle gives protection, but it does not grow. So for an arthropod to grow it must shed it and build a new one — which is why a crab or an insect moults again and again.